With the 306-horsepower 3.5L V6 engine, 6-speed shiftable automatic transmission, and rear-wheel drive, the 2008 Lexus IS has been averaging 24.88 MPG (miles per gallon).

The 2008 Lexus ISs in this analysis were driven mostly on the highway at an average speed of 66 miles per hour (about 40 percent of the miles driven). The rest of the miles were in the city (19.8 percent), in heavy traffic (15.8 percent), and in the suburbs (0 percent).

In addition, the average 2008 Lexus IS in this analysis was driven on hilly terrain with a medium foot and the AC not used at all.

My real world economy has been 33 mpg hwy (not the computer figure, an optimistic 34.5-35)which is better than expected and slightly over 20 in city driving (not heavy traffic), also better than expected
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2008 Lexus IS Real Gas Mileage: Cons

Year

Body/Powertrain

Comment

2009

4dr Sedan 204-horsepower 2.5L V66-speed shiftable automatic RWD

Wish it got better fuel economy for such a small car and engine... Sticker is 21/29, I'm averaging 27MPG (computer read); avg on hwy is about 33-34 mpg while city is about 19-23mpg. Engine variety would help, a 4 cyl + turbo or something, but toyota/lexus isn't exactly known for that.
see full Lexus IS review

2008 Lexus IS Gas Mileage (MPG)

Unlike other fuel economy surveys, TrueDelta's Real-World Gas Mileage Survey includes questions about how and
where a car was driven. So you can get an idea of the Lexus IS's
real-world MPG based on how and where you drive a car.